1. Field of the Invention
This invention pertains to the gasification of carbonaceous solids and is particularly concerned with the fluidized bed gasification of agglomerating coals.
2. Description of the Prior Art
The formation of agglomerates is a problem frequently encountered in the fluid bed gasification of caking coals. This is caused by plastic properties which develop when such coals are subjected to temperatures above their softening point. Upon reaching this point, generally between about 700.degree. and about 900.degree. F., the coal particles begin to swell and deform due to the formation of bubbles during devolatilization. As the temperature increases, deformation becomes more severe, the coal becomes plastic and sticky, and may eventually become fragile. The sticky particles tend to agglomerate and form large clusters which interfere with operation of the fluidized bed. The fragile particles break into fragments which are further attrited to produce fines. These fines are entrained in the rising gases and carried overhead from the bed. The fines may contain from 10 to 20 percent of the carbon fed to the process. The loss of this carbon before it can be gasified decreases the efficiency of the process.
The traditional method for dealing with fines produced during gasification operations is to separate them from the product gas and return them directly to the gasifier. This technique, however, is generally unsatisfactory because the returned particles are rapidly entrained in the product gas and carried overhead from the fluid bed at a velocity approximately equal to that of the gas. The residence time of the fines in the reactor is therefore quite short and little additional carbon is gasified.
Several methods have been devised to alleviate agglomeration problems encountered when caking coals are gasified. These involve low temperature carbonization in a pretreating zone, carbonization at progressively higher temperatures in a series of fluidized beds, injection of the coal into the gasifier as a very fine powder, or passage of the coal through a free-fall pretreatment zone in the presence of steam and oxygen. Although these techniques can be useful under certain conditions, they all have pronounced disadvantages. They are either expensive, result in the loss of valuable volatile constituents from the coal, or require the use of complex equipment.
An additional method which has been proposed for controlling agglomeration involves withdrawing char particles from the fluidized bed gasifier, mixing these with raw feed coal of comparable size, and injecting the char particles and coal into the fluidized bed. This procedure is intended to prevent sticky feed coal particles from coming in contact with one another to form agglomerates but large amounts of char must be injected into the fluidized bed with the coal if it is to be effective. Unless the ratio of char to feed coal is large, the probability of coal particles contacting one another to form agglomerates will be very high. Normally, a weight ratio of char to feed coal ranging from 10:1 to 30:1 is required to insure that agglomeration will not interfere with the continuous operation of the gasification process. Moreover, the average particle size of the char will be approximately the same as that of the feed coal. The shear forces in the turbulent fluidized bed are often sufficient to break the relatively weak bonds between the sticky coal particles and devolatilized char particles of comparable size and hence some agglomeration may occur even though the char is present.
Although the technique referred to above may aid in controlling agglomeration, it has pronounced disadvantages. The necessity for withdrawing large quantities of unconsumed char from the fluidized bed gasifier, mixing it with the coal, and then returning the combined stream to the gasifier limits the amount of coal that can be injected into the gasifier, necessitates the inclusion of equipment for withdrawing and handling the hot char particles, reduces the overall char gasification rate in the system, tends to decrease the thermal efficiency of the process, requires the use of higher fluidizing gas velocities than might otherwise be required, and may give rise to other difficulties. As a result, this technique leaves much to be desired.